Floating plastic screen



Jan. 6, 1959 Filed Sept. 26, 1955 A. CHAM PAG NAT 2,867,346

FLOATING PLASTIC SCREEN 2 Sheets-Sheet l FIG. 1

wyda, H Tra le Ney s Jan. 6, 1959 A. CHAMPAGNAT FLOATING PLASTIC SCREEN2 Sheets-Sheet 2 Filed Sept. 26, 1955 /gl/E/VTOEC rrae/vsrs niteclStates FLoATING `PLASTIC SCREEN Alfred Champagnat, Paris, Franceassignorof one-half lto The British Petroleum `Company Limited, London, England,a Britishcompany This invention relates todevicesfor theprotection ofvolatile and non-volatile*liquidsstored in reservoirs, and relates moreparticularly :to floating screen or blanket devices for protectingsuchliquidsstored in tanks, particularly tanks of the fixed-roof type.vThis .invention is of utility especially intank storageof volatileliquids whose vapor pressure at the storage temperature doesnot exceedatmospheric pressure, particularly such liquids as petroleumrdistillates, crude 'p etrol, vand volatile alcohols, and is of utilityalsoin tankstorageof wine and other liquids, volatile or :non-volatile,which undergo alteration inthe presence of air.

Objects and advantages of the invention will be set forth in parthereinafter. and;in partwillbe obvious therefrom, or maybe learnedbypractice with the invention, the same being realized `and attained bymeans of the combinations, vimprovements andinstrumentalities pointedout in the appended claims.

The invention'consistsgin the novelparts, constructions, arrangements,combinations and improvements herein shown and described.

The accompanying drawings, referred to herein and constituting` a partihereof, illustrate. several ,embodiments 4of the invention, and togetherwith the description, serve to explain the principles of the invention.

The present invention hasf0r its lobject an improved device for theprotection of liquids storedin reservoirs and, more particularly, `aydevice principally intended to reduce losses by evaporation in storagetanks for volatile liquids whose vapourpressure at thestoragetemperature does not exceed atmospheric pressure.

A furtherl object of the invention is1to provide means to ensure a goodseparation between the ambient air and the free surface of the:storedliquids so as to avoid an alteration of the latter.

A further-object of the inventiongis 1a device intended to reduce in aconsiderable degree therisk -of `explosion and re during the `storage ofinammable volatile liquids.

A still -further object of the invention is a device intended tocooperate with an existing cylindrical ,reservoir with vertical axis,.ofcircular ornon-circular cross-section, of the xed roof type (conicalroof), `withoutthe necessity of dismantling orvmodifying Vthe latter,and to reduce considerably `the losses by evaporation of the storedliquids and the risk of re vifthe liquids are inammable.

Accordingto van essential lcharacteristic of theinvention there ismadev,tofloatv/onvthe free` surface of the liquid stored in the reservo-irwith xed roof la light flexible screen .(blanket),made ,ofaplasticlmaterialvunattackable by the stored liquid, the surfaceor vthescreen being a little smaller than the cross-section `of `the reservoirand provided at its periphery with a vertical apron also lmade, ofplastiematerial.

lt, has beensugg'ested already to reduce vthe losses by evaporation inVreservoirs with lixed roof `llvyvarranging a screen on the liquids.

For example, it has been suggested to make Vto float on the storedliquids devices similar. to -oating roofs constructed wholly or partlyin metal `and therefore rigid.

It has 4also been proposed to arrange on'floats,.varyi.ng in numbers andconstruction, 'flexible :oating .screens made, for example, of fabricsimpregnated witlsynthetic resin, the whole floating on theliquidtobestored.

The known devices wereintended eitherto oatdirectly on the liquid phase`without a vapour phase ,beinginterposed between the surface lof thevliquid .andthe screen, or they-were arranged at a certainV distance.abovelthe liquid, with a vapour phasebetween the free surface of theliquid and the rfloating screen, and the communica- Ition between thatvapour phaseandthewall of the reservoir being closed so Yas to preventthe vapours ofthe liquid from escaping at the periphery of the floatingv screen.

ln all embodiments suggestedup till now, .the Vscreens or analoguedevices pressed stronglyv againstthe vertical walls of the reservoir,either ,directly or by the intermediary of a joint made ofelastic orrigid material.

Now, it is well known lthatthe.innerwalls of `storage reservoirs withvertical axisy are not inthe form of geometricallyperfect cylinders;their vertical walls .are formed by rings joined by riveting orsoldering whereby .projections at the innersurfac'e of thevcylinder arecaused. Moreover, the'sheet metal hoops ofthe reservoir often areinclined towards the vertical so that they may interlock, in consequenceofwhich ,the diameter of `the reservoir is not ystrictly constant at alllevels.

`Experiments carried out ,bythe Aapplicant have .shown that floatingscreens whichpress toostronglyagainst the walls of the .reservoir arecaughtor wedged .by .thev unevenness or irregularities of the walls.when the level .of the liquid is changed.

For this reason, ,according to an essential .characteristic of theinvention, the new floating Vscreen .conceived kkby the applicant has adiameter distinctly smaller than the diameter ofthe reservoir so as toleave on its periphery a clearance of a magnitude such thatsubstantially all catching and wedging by the unevennesses of thereservoir walls is avoided when Vthe screenrises, or'falls with thelevel of the liquids to be protected. In general, a clearance ofat-least 2O mm. will be found Ysufficient. for `the purpose although agreater or less clearance maybe required or be feasible, as the case-may be, in some installations.

Moreover, the oatirng blanket according to the invention is made of aflexible elastic material in the form of sheets or lilms-which are verythin, Which/is not seriously attacked by the stored liquid, .-issufciently flexible to -be deformed whensubmitted to stresses resultingfrom being caught by the unevennesses of the inner walls of thereservoir, and is sufficiently elastic to resume its original form whenit is past theunevenness. lnthe usual clase of storing hydrocarbons andalcohols the material .used to manufacture the blanket according to theinvention is ,a llexible elastic synthetic resinor' polymer, sometimes.here inafter referred to as "plastic material,'in theforrn of sheets orlilms which are very thin and is, ,preferably pure, that isnonfplasticized, ypolyvinyl .chlorideiin the form of sheets` or .lms.having a thickness `in the range of from some hundredths to some tenthsof a millimeter; however, `the blanket Vmay also be made kof other.'plastic materials such as polyvinyl chloride-polyvinyl acetateVcopolymers, other vinyl co-polymers, vinylidene chloridesuperpolyamides, .uorinated .polymers ,such as Vpolytetranuoro-ethylene`(comnlfer'cially being ymarketed las .-Tef 1on) andpolymonochlorotriuro-ethylene (tluorocarbon), `and polyethyleneterephthalate Aand other ,polyester ,resins, lwith similar chemical,properties provided that these materials are not yseriously attacked by.the

aser/,34s

- 3 stored liquids. The fiuorinated polymers, in particular, areresistant to temperatures appreciably exceeding 100 C. which allowscleaning of the reservoirs with steam whichis easily available inpetroleum refineries.

To ensure the fioating of the blanket on the liquid to be` stored, it isprovided, according to another characteristic of the invention, withfloats made of the same or a similar plastic material and `attached inthe form of hollow cells to the lower surface of the blanket.

Since the floating blanket according to the invention leaves a freeannular liquid surface beyond its periphery, it is, 'according toanother characteristic of the invention, fringed with an apron, which ingeneral is substantially vertically arranged and has a height of amagnitude so as to maintain, above the free annular liquid surface andat the walls of the reservoir, a circular ring of gas in equilibriumwith the stored liquid whereby the losses byevaporation on that freeannular surface are restricted in a considerable degree. In general, anapron height of at least 200 mm. is preferred although a greater orlesser height may be found suitable for particular installations andparticular materials. In effect, the volumeof the annular spacementioned is filled with air practically saturated with the vapours ofthe stored liquid whose density is greater than that of air withoutvapours'so that the evaporation of the stored liquid is impeded, and theonly possible losses are due to the diffusion of the atmosphere of theannular space into the vapour phase of the reservoir. -Now, it is knownthat the diffusion of gases is a much slower process than evaporation.

The experiments of the applicant have shown that the losses byevaporation are reduced to about a fifth or a tenth of what they wouldotherwise be, by the use of the vertical apron mentioned above.

Moreover, according to an additional characteristic of the invention, itis possible to counteract the diffusion of the gases between the annularspace and the vapour phase of the reservoir vby closing, more or lesscompletely, the annular space at its upper part. This closure may beeffected by an appropriate shape of the vertical apron, for example bygiving the apron the form of a Z or of Aan inverted L. This shape isfeasible thanks to the construction of the apron by a film of plasticmaterial of very slight thickness to 20 hundredths of a millimeter).Such a film is, in fact, suiciently deformable and 'elastic to offer noappreciable resistance when it encounters a ring of rivets, for example,in the course of variations of the liquid level.

There is thus effected, according to the invention, even if the verticalapron has not the form of a Z or of an inverted L, but even more so ifit has this shape, a gaseous seal between the periphery of the oatingblanket and the wall of the reservoir. This gaseous seal is infinitelydeformable and contact between the edge of the blanket (or the upperpart of the apron) and the wall of the reservoir is only made along aline, or zone, of very slight thickness, practically equal to thethickness of the lm of plastic material (some hundredths of amillimeter). l Y

Moreover, since the diameter of the blanket is distinctly smaller thanAthat of the reservoir, contact is never made over the whole periphery ofthe blanket so that the surface of the total zone of contact (length ofthe line of contact multiplied by the thickness of the film) is furtherreduced.

Since the line or zone of contact is extremely flexible, one does notmeet, in accordance with the invention, any of the disadvantages of theearlier proposed devices, such` as catching, etc., and the gaseous sealachieved` according to the invention restricts in an effective mannerthe losses by evaporation.

In the preferred embodiment of the' invention there is `ittached to theblanket a at metallic element, electric lly connected to the material ofthe'reservoir. One

may, for example, provide on the lower surface of the blanket a metallicnetting covering that surface completely, or at the upper surface of theblanket a thin metal sheet, the netting or metal sheet being connectedelectrically to the material of the reservoir by a flexible conductingwire.

In this Way there is obtained an electric blanket which l collects andcarries away to the material of the reservoir the electrostatic chargesproduced through the pumping of the non-conducting liquids, such as thehydrocarbons, and the production of sparks which otherwise could causeexplosions in the gaseous phase is thus made impossible.

There will now be described, in order to illustrate the possibilities ofcarrying out the invention, without thereby in any way restricting thescope of the invention, embodiments shown by way of example andrepresented in the attached schematical drawing as follows:

Fig. 1 is a vertical section through a reservoir with fixed roofequipped with a floating blanket according to one embodiment of theinvention.

Fig. 2 is a view on a larger scale of part of the fioating blanketaccording to another embodiment of the invention.

Fig. 3 is a section along the line III-III ofk Fig. 2.

Fig. 4, nally, is a vertical section of part of the oating blanket withvertical apron according to still another embodiment of the invention.

With special reference, in the first instance, to the ernbodiment ofFig. 1, it is seen that a device according to the invention essentiallycomprises a floating blanket 1 on the surface 2 of a volatile liquid 3contained in reservoir 4 with fixed (conical) roof 5 provided with avalve 6 for filling and emptying and a respiratory orifice 7.

In its preferred form for use with hydrocarbon liquids and alcohols, theblanket 1 comprises, preferably, a continuous sheet 8 of pure, that isnon-plasticized, ployvinyl chlorideof a thickness which, preferably,.isin the range of from about 10 to about 20 hundredths of a millimeter.Blanket 1 is supported by hollow fioats 9 also made of a. very thin filmof pure polyvinyl chloride, as will be; explained below.

The level 2 of the liquid is always situated below theupper sheet 8which constitutes the blanket in the proper4 sense. The vapour phase 10between floats 9 and below the upper sheet 8 has, preferably, athickness of thev order of a few millimeters. Stated otherwise, theupper' sheet 8 is maintained by means of floats 9 at a distanceof lessthan half an inch above the free surface or level 2 of the storedliquid.

The blanket 1 is of a diameter distinctly smaller than the diameter ofthe reservoir 4 and is fringed aroundits periphery 11, preferably, witha vertical apron 12 wh1ch leaves an annular clearance space or interval14 of at least 20 mm. span between that periphery and the lnner wall 13of the reservoir. The height of the Vertical apron 12 is, preferably, atleast 200 millimeters.

The annular space 14 comprised between the vertical wall 13 of reservoir4 and apron 12 which fringes blanket 1 is thus constantly filled withair saturated with the vapours of the stored volatile liquid 3. Thegaseous ring-shaped space 14 behaves as an infinitely deformable sealwhich takes the exact shape of the wall 13 of the reservoir when thelevel 2 of liquid 3 is varied by filling or discharge through valve 6,and evaporation into the main Vapour phase 15 of the reservoir isimpeded by the presence, above the free surface 2 of liquid 3, of thatgaseous .space 14 saturated with the vapours of the volatile liquid.

As shown above, the passage of the vapours of the volatile liquid 3 intothe vapour phase 15 of the reservoir is now only still dependent on thediffusion of the molecules of the volatile liquid 3 out of the annularspace 14 'which diffusion is a much slower process than evaporation,

The effect `of the Vfloating v.blanket `with .its .vertical apronis thusto restrict considerably,therateat .which the vapour phase ofthe-reservoir .issenriched with lthe vapours Vof the stored liquid .3.An `equilibrium .establishes itself between the emission of vapours :of

liquid-3 above the vertical apron 12 andtheloss of air, charged withthese vapours, during eachdaily-respiration of the vreservoir through-the respiratory gorice 7, in such a manner'that theconcentrationfof-thevapours of the stored liquid in the vapour phase 15of the reservoir is always muchlowerthan-.would be the con- On the otherhand, it is knownvthat whenl aureser yvoir is filled withan'electrically non-conducting liquid (for example with hydrocarbons)the friction of the nonconductiug liquid on the walls of the fillingpipe line causes the non-conducting liquid to be charged with negativestatic electricity, and that lthis phenomenon of l staticelectriiication is the cause of numerous tires or explosions which occurin reservoirs or inflammable products, such as petroleum products.

It is possible to reduce to aminimum and even to eliminate the danger ofire and explosions by fixing, accordingtoan additional characteristic ofthe invention, on the lower surface of the blanket, by any suitablemeans, for example, by metal hooks 16a going through blanket 1, a metalnetting 16 which covers the whole lower surface of the blanket. Thisnetting 16 is electrically connected to thematerial ofthe reservoir byat least one flexible conducting wire 17 of copper or other suitablyelectrically conductive material, of sufficient length to allow thedisplacement oflblanket 1 between its upper position, when the reservoirisfull, and its lower position, when the reservoir is empty. In thismanner there is achieved an electric screen connected electrically tothemetallic roof of reservoir 4. This screen 16 collects the whole of thedangerous electric charges present on the surface of liquid 3 anddischarges them to the material of the reservoir from which they may owto ground.

The vapour phase 1S of reservoir 4 where explosive gas mixtures mayaccumulate is thus completely protected against electric charges whichmight cause sparks so that all dangers of explosion or tire due tostatic electricity are absolutely eliminated.

It will be understood that it is possible to achieve the same protectionby arranging, instead of the metal netting 16 attached to the lowersurface of the blanket, a netting or, for preference, a thin flexiblemetal sheet xed to the upper surface of the blanket and electricallyconnected to the material of the reservoir in the same manner as in theembodiment represented in Fig. l.

In any case, it is a matter of using a plastic material obtainable inthe form of Very thin films which alone allow to achieve the flexibilityand elasticity desired for the blanket according to the invention. Inthis manner there is obtained a very light blanket which olers thefurther advantage that it may be constructed andtransported in sectionsto the reservoir Where it is to be installed. The sections of theblanket can be introduced into the reservoir through a manhole andassembled in the reservoir without any alteration of the reservoir.

In the construction of the sectional blanket of this invention, forpreference are used thin sheets of a plastic material which isindustrially. produced in the form of very long ribbons, about 0.70 to1.0 m. wide. Itis then easily possible to make a circular blanket,according to Vthe invention, of suitable diametenxby assembling these,ribbons orbands` placed sideby sidegand thencutting ,the rim in'`circular form.

`but they may also form an integral part of it which results n theblanket floating by itself (auto-floating blanket) without separatefloats being attached to it.

In Figs. 2 and 3 is represented a particular embodiment of anauto-floating blanket 1 made from individual bands 31 of plasticmaterial assembled as at 32 with sliding fasteners 33.

In the embodiment represented in Figs. 2 and 3, a iilm 30 .of plasticmaterial having a great number of lsquare cells 29' in its surface ishermetically sealed, as

by welding or glueingaround the rim 28 of each cell, to the lowersurface22 of the blanket lilm 34 so as to provide the floats 9 `lilled withair. These cells 29 are obtained for example by forming depressions inthe, film 30 of polyvinyl chloride or of another plastic material 'ofthe same kind as that of the blanket film 34.

Such lauto-floating sectional blankets may be mass produced fromindividual bands 31 of the plastic material, of great length and a widthL of, for example, 0.70 to 1.0 m., which then are assembled as at 32 lbysliding fasteners 33 as will be described below.

The oating blanket 1 may also be constructed (see -Fig 4) by using, asindependent floats, square or rectangular elements obtained, forexample, by the process mentioned above, `that is by welding or gluing afilm 34 of plastic material onto another film 30 of plastic `material inwhich latter there have previously been vacuum moulded a great number ofsmall cells 29.

`Sections of the assembled flms30 and 34 l(which sections are in generalsmaller than thebands 31 in the construction) are adhesivelybonded `orotherwise fixed at 35 under a third lilm 36 of plastic, material whichconstitutes the upper surface of blanket 1.

The transport of the floating blankets according to the invention andtheir introduction into the inside of the reservoir, and also themounting of the blanket in the reservoir-,are tasks which-may easily becarried out in the following manner:

The floating blanket 1 with its cellular :floats 9 fixed at itslowersurface 22 may be rolled like a tobacco leaf forming a cigar, .so as tofacilitate its transport and to allow its introduction, throughamanhole, into the reservoir. However, Vin the` case of reservoirs witha. greater diameter Ithan about 8 m., the length of the rolled blanketalso exceeds 8 m. which makes it diflicult to transport, Moreover, themanholes of the reservoirs have a diameter of the order 70 cm. whichdoes not allow the introduction of rolled blankets of verygreatdiameter. `For these reasons it is necessary to separate the oatingblankets into sections not exceeding the dimensions of about 8 m. by l5rn., these sections, constructed in advance, being transported in theformV of rolls and introduced through the manhole vinto the reservoir tobe equipped.

According to one characteristic of the invention, in order to makepossible a rapid and eicient assembling` of these elements (as well asthe assembling of the bands of width L in the construction of theblanket) there are provided sliding fasteners' 33, with or Withoutlhooks, along the assembling linewhich allowsto carry out .the assemblingof the various parts or of ythe .sections inside the reservoir 4 easilyand morey quickly.

Thesliding fasteners 33` are preferably withoutlhooks and made of aplastic material of the same kind asthat of oating blanket 1. They arerelativelyimpervious to the liquids or gases under slight pressure -butit is advantageous to improve the imperviousness of these fasteners bycoating the plastic material used with an adhesive varnish, or betterstill by covering them with an adhesive ribbon made of a fabric or aplastic material coated with such a varnish.

It has been indicated previously that the effectiveness of the floatingblanket must be completed by attaching, along its periphery 11, an apron12, of generally vertical form, which apron also is made of plasticmaterial.

Apron 12 may be fixed to periphery 11 of the blanket by welding,cementing, by sliding fasteners, or any other means. It may be of anysuitable shape so as to maintain, between the periphery 11 of theblanket and the walls of the reservoir 4, a circular space 14 iilledwith air saturated with the vapours of the stored liquid 3. lt may befixed in advance on the circular rim 11 of the sections of blanket 1, orelse it may be fitted when the blanket has been mounted inside reservoir4.

The vertical apron 12 may with advantage be equipped with reinforcingelements, mouldings, or similar means so that it may rapidly take onagain its generally vertical form if, 'by accident, it should bedeformed, for example by being caught by the unevennesses of the wallsof reservoir 4.

It is, of course, necessary to increase the number and/ or the volume ofthe floats 9 arranged under the periphery 11` of blanket 1 to balancethe weight of the vertical apron 12. For the sake of simplification,this increase in the number and/or volume of the floats is notrepresented in the drawing.

In a preferred embodiment represented in Fig. 4 the vertical apron 12consists of a film 38 of polyvinyl chloride fixed to the periphery 11 ofblanket 1 by means of flexible angle pieces 39 made of plasticizedpolyvinyl chloride with welded joints 39. The film 38 is very thin to 20hundredths of a millimeter thick) and would not maintain its verticalposition unless reinforced, as shown, by another film 40 which is alsovery thin. This film 40 may with advantage be folded back to form aflange 41 which closes, more or less completely, the gaseous ring 14comprised between apron 12 and the walls of reservoir 4.

Films 38 and 40 are fitted together with press buttons, or better bymeans of bolts and screws 42.

Film 40 has stiffeners 43, placed at regular distances, which preventthe whole of apron 12 from inclining towards the interior-of thereservoir, or in case the position should have been changedaccidentally, for example by the effect of certain unevennesses, allowthe apron to take up again its former position. Film 40 may also bemoulded finto a corrugated form so as to have a greater elasticity withrespect to deformations.

As regards the construction, it may be noted that it is advantageous toequip the blanket, while it is 'being constructed, with the fixed film38 which is folded and rolled together with blanket 1 for the transportto the reservoir, while film 40 is easily fitted on blanket 1 when thelatter is already in place in the reservoir and is thereby provided withthe necessary hold.

In order to allow gauging and sampling in the reservoir it is desirableto provide in the floating yblanket at least one orifice 44 (Fig. 1) ofa diameter sufficiently large to allow the passage of the instrumentsfor gauging and sampling. Such an orifice 44 is placed exactly below thevent 45 provided in roof 5 of reservoir 4 to allow gauging and sampling.Each orifice 44 is preferably bordered by a skirt 46 which ispractically vertical and is intended to maintain at the free surface 2of liquid 3 an atmosphere of air saturated with the vapours of liquid 3.ThisV skirt 46 has the form of a double cone to allow the introductionand the withdrawal of the sampling vessel without the latter beingcaught at the walls of orifice 44,

aser/234e ample, by stretching two vertical cables 47 betweenv roof andbottom 48 of reservoir 4. These cables 47 pass through the blankets byholes 49 provided for the purpose. These holes 49 are bordered by'acollar 50 also of polyvinyl chloride so as to restrict losses byevaporation.

Moreover, it is desirable to provide in the floating blanket holes 51 ofvery small diameter to allow the drainage of water of condensation whichmay accumulate on blanket 1.

In order to prevent the vapours of liquid 3 enclosed below blanket 1from escaping through holes 51, there are provided, with advantage,tubes 52 of exible plastic material, soldered below the blanket andabutting upon discharge holes 51, which tubes 52 are immersed in thestored liquid 3.

When reservoir 4 is provided with a swing line, there is with advantageproviled in 'blanket 1 a groove of suit able length (not shown in thedrawing) intended to allow Vthe passage of the swing line. Such a grooveis bordered on each side by a vertical apron to restrict evaporation.

The following are examples of various ways of carrying out thisinvention:

Example 1 A plastic floating screen of pure polyvinyl chloride wasconstructed for a riveted reservoir having a diameter of 12 m. and aheight of 12 m. intended for storing a stabilized gasoline of a densityof 0.720 at 15 C.

The pure polyvinyl chloride used had the following characteristics:

Specific gravity 1.4. Tensile strength 8,000 p. s. i. (pounds per squareinch). Compressive strength 11,000 p. s. i. Flexibility strength 14,000p. s. i. Izod impact 0.8 ft./lbs. per inch in notch. Elasticity modulus300 kgs/sq. millimeter.

The screen consisted of 17 strips 70 cm. wide of film of a thickness of20/ 100 mm. in thickness. Under said strips there were welded plateswith hollow cells (alveoli) having 560 alveoli of 9 cm3 per squaremeter, 10 mm. deep, constructed, as in Figs. 3 and 4 of molded andwelded film of 15/100 mm. thickness. The strips were 'assembled inadvance edge to edge by means of sliding fasteners made of polyvinylchloride without hooks.

The vertical skirt or apron was constructed as in Fig. 4; the part ofthe skirt 33, made of film of 15/ 100 millimeter, was welded in advanceonto the periphery of the screen to be transported with it.

For purposes of transportation the screen was divided into two elementsby opening the sliding fastener which went through a diameter. The twoelements were then rolled up in the form of a cigar 6 meters long, witha diameter of 0.5 meter each weighing 60 kgs. The two rolled up elementswere introduced into the reservoir through a manhole 0.70 m. indiameter. They were then assembled in the interior of the reservoir byclosing the sliding fastener which had been sealed by applying anadhesive insoluble in the gasoline. Previously a metal grid 11.90 m. indiameter had rbeen introduced into the reservoir which grid was fixedunder the plastic screen by means of metal hooks. This grid shown at 16Fig. l is the electric screen for the elimination of static electricity.it was grounded to the reservoir by means of 3 flexible cables 17.

The mounting of the vertical skirt of Fig. 4 was completed in thereservoir by fixing by means of bolts 42 the part 40 of polyvinylchloride lm elements having a corrugated form and being of a thicknessof 100 mm. Thereupon the supplementary devices were inasomansta1led.,-viz. :double funnel 46 .for gauging and sampling, andthecables :47 forpreventing thescreen from turning.

The installation took 20 hours and occupied 2work men. The plasticfloating screen thus constructed and nstalledleaves a clearance of from`2 to 3 cm. between its periphery and the wall 'of the reservoir, whichlatter is not a perfect cylinder on -account of its being constructedwith superposed hoops of sheet metal.

The following merits-characteristic features-(not counting the metalgrid which weighs -300 gr. per nmz):

LWeight per square-meter 1 kg. JApparent'density -0.2 kg. Floatability(per square meter on gasoline of-a density of 0.7) 3100 kg. (whichit'can sustain before sinking).

yAfter thefgasoline-has been-introduced `into the reservoir the screenfollows the variations of its level, the superior :film ofthe screenbeing 5 mm. -above .said level, without clinging in a troublesome-way tofthe; rows of.rivets, and the operationsv .of gauging and `sampling;proceed inthe .usual way. i l

YThe losses`by `evaporation'in the` reservoir-equipped with thisfloating screen and in an identical control reser- -voir without screenthe two placed side by side, have been .measured The two reservoirs werefilled with the same 4,gasoline of density 0.7.20 at 1'5 C. and .Reidvapor pressure of V0.650"l g./ cm.2 at 100 F. and they were emptied in a.parallel manner in one month according to the needs of operation. Themeasurement vof the losses was eiectedby two "methods 3.(a)`Determination f the hydrocarbon .content of the .vapor phase ofrthe tworeservoirs during a month at a mean temperature of 1'8 'C.

`If. estimatedby. thisme'thod .lthe losses. are thus reduced =by 83.5%.

(b.)lDeterminationfofbutane:in .the gasoline of the two'reservoirs -atthe.beginning andthe end of the storvage .of 1 monthfthe reservoirs.being full at .the -be- L ginning and almost empty-at the end Reservoirvt ith Control -Screen reservoir IBeginning of storage, butanepercent..." 6. 0 6. 0 After 30 days, butanepercent 5. Q5 5. 6 Losses 'ofC4 0.05 0. 4

'The saving calculated by'this method amounts to 87.5%.

Example 2 A 4floating plastic screen 'of pure polyvinyl chloride "wasconstructed for a rivetedreservoir 15 m. in diameter and m.high-*intended to ystore `crude Kuwait *petroleum The polyvinyl chlorideused had the same character- 'istics asin Example 1, and theconstruction was identical. They screen hadv 22 @strips 0.70 rn. wide.There was fino electric screen for crudefpetroleumiis sufciently con-*du'ctive to' avoidbe'ing' charged electrically. The oatingv screen wastransported in two rolls 7.501meters long and 'ofl-a-diameter ofi0.65Imeter,1each: weighing `90 kgs.

.t The .insertion into'ithe. .reservoir :through a.: manhole andthejmounting.ofthe.` screen were vaccomplished as in Ex- -`"ample l.`iTne workn1equi1'edf30lhourstandi.workmen e -the parts of the floatingplasticscreen.

10 The clearance between the periphery and the wallet' the reservoirVaries between .3 .and 4 cm. The float- .ability of the screen is3.6kgs. per m.2 on crude petroleum of a density of 0.865 at 15C.

The Reid -vapor pressure of the crude petroleum stored ibeing 0.510kg./cm.2 at 100 and the mean temperature in the course of the storingperiod being 7 C., the losses by evaporation in the reservoir providedwith the screen were compared with those in another reservoir withoutscreen the two reservoirs'being-V full of the same crude petroleum.

The average of the hydrocarbon concentrations of the vapor phase during12. days was 0.8% in volume in the reservoir with screen and 5.7% in thereservoirwithout screen.

The reduction yofthe losses by evaporation had thus been 86%. Moreoverthe odor of hydrogen sulfide was extremely attenuated through thepresence Iof the lioating screen.

Example 3 A welded reservoir 3 m. in diameter and 7 m. in height usedfor storing Ynon-rectified ethyl alcohol-was equipped with a floatingscreen of vinylidene chloride.

The vinylidene chloride used had the following charac- Theconstructionwaslidentical with that of Examples 1 and 2, `butilms of 20/100mm.were-used for all No electric screen was lset up, for alcohol is 'notlelectriieclby pumping.

The screen, weighing 8kg. was transported rolled up as a single'part 3m. long and 0 40 rn. in diameter. It

was inserted in the reservoir througha manhole and the mounting ofthewverticalskirt was completed.

The work took' 2 hours .with-a single workman. It was found possible toreduce the clearance at `the edge of the screento 'l cm. on account ofthe welded Evaporationlosseswere measured by comparing them with thosein-anidenticalcontrol reservoir located next to the first,.saidgtworeservoirs being half filled with -the same non-rectifiedalcohol.

AfterS days ofstorage atatemperatureof 22 C. in the liquid theconcentration-of alcohol vapors of the vapor phase .ofthe reservoirwithscreen was 0.26% as against 6.2% in the control reservoir, so thatthe reduction of losses by evaporation attained,96%.

Example 4 A-riveted reservoir, 4.5 rnetersnin diameter and 7 rnetershigh, intended for .the storage Lof benzol` was equipped..with afloating plastic screen of polyethylene terephthalate having the.following characteristics:

Specific gravity 1.3 Tensile strength p. s. vi 21,000

The construction of the screen wasjthe same as that ofthe `screensdescribedbefore, but the assemblies were effected by cementing insteadofwelding, becausepolyethylenewterephthalate ishard to weld. The upperface .of-.thexyscreen consisted of ya lilm .of,15/100..mm. and

the hollow .alveoliof'tilm of 10/ 100 mm.

il. assembled' edge to edge by cementing. The vertical skirt ofV/100 mm.film was likewise cemented. Ihe screen rolled into a cylinder 4.50meterslong, V,vt/"eighing 8 kg., was inserted and mounted in thereservoir and its installation was eiected in 5 hours by two workmen.

The clearance at the edges ofthe screen is 2 cm.

Its characteristics are:

Weight per m.2 0.500 kg. Apparent density 0.1. Floatability on benzol3.6 kg. per square meter.

The invention in its broader aspects is not limited to the specificcombinations, improvements and instrumentalities described butdepartures may be made therefrom within the scope of the accompanyingclaims 'without departing from the principles of the invention and witnout sacrilicing its chief advantages.

What is claimed is:

1. The combination with a Storage vessel for volatile liquids, thevessel having an upright wall and a fixed roof, of:` a -blanketcomprising an upper thin sheet of liexible elastic plastic material andattached to the underside of said upper sheet, a plurality of small,contiguous, cellular oat units, said float units comprising an upperthin sheet of flexible elastic plastic material having attached theretoanother thin sheet of exible elastic plastic material in which aplurality of contiguous hollow cells are provided, these Ytwo oat unitsheets being attached one to the other around said cells to seal saidcells individually and provide said plurality of small contiguouscellular iioat units, said blanket being capable of floating on the freesurface of the stored liquid with said first mentioned upper thin sheetat a small distance from and in a parallel direction with the freesurface of the stored liquid for protecting the same againstevaporation, the blanket being of a size and configuration to coversubstantially, but not entirely, the entire free surface of thevstoredliquid so as to leave only a narrow clearance lbetween theperiphery of the blanket and said wall providing a narrow free annularliquid surface between said periphery Yand said wall whereby catchingand wedging by unevennesses of said wall is substantially avoided whenthe blanket rises or falls with the level of the stored liquid; and, anapron for said blanket, made also of at least one thin sheetl offlexible elastic plastic material, which is of suiicient rigidity to beself-supporting yet suiciently flexible to be deformed if submitted tostresses resulting from being caught by unevennesses of said'wall, andsuiciently elastic to resume its original form when it has passed theunevennesses, said apron extending upwardly from said blanket andfringing the blanket around substantially its entire periphery insubstantially wholly spaced relation to said wall for maintaining, abovesaid narrow free annular liquid surface and at said wall, a ring ofvapours in equilibrium with the stored liquid, whereby the losses byevaporation on the free annular liquid surface are restricted inaconsiderable degree.

2. A device as specilied in claim 1, in which the flexible elasticplastic sheet material of both the blanket and the apron isunplasticized polyvinyl chloride.

3. A device as specified in claim 1, in which the iieXible elasticplastic material of both the blanket and the apron is a polyamidesynthetic resin.

4. A device as specified in claim 1 in which the blanket is sectional,being divided into a plurality of individual bands disposed side byside, each band comprising an upper strip of said exible elastic plasticsheet'material,

sliding clasp fastener connecting means, provided along each edge of theupper strip of the band which is contiguous with an edge of an upperstripY of an adjacent band, for connecting contiguous edges, andattached to the underside of the upper strip, a plurality of sealed.float units, said oat units comprising a strip of said flexible elasticplastic sheet material having attached theretoV another stripofsaidflexible elastic Vplasticsheet material in which aplurality of`small, contiguous hollow cells are provided, the two float strips ofeachsaid band being sealingly attached one to the other around said.cells to seal said cells individually and provide'said plu rality ofsealed oat units.

5. A device as specied in claim 4, in which upper 7. A device asspecified in` claim '1, in which saidV cells are disposed in an alveolarmanner.

8. A device as specified in claim 1 including a metallic element formingan Velectrical screen attached to the blanket and connected to ametallic part of the reservoir by at least one electrically conductingwire.

9. A device asspecilied in claim 1 in which said apron is provided withan annular ange at its upper edge, which substantially completely closesthe upper part of the annular space contained betweenthe walls of thevessel and the apron, to substantially reduce the losses of vapours bydiffusion.

10. A device as specified in claim 1, in which the apron comprises acylindricalrsheet of the flexible elastic material attached to theblanket, and a plurality of reinforcing members of liexible elasticplastic sheet material of greater thicknessthan said cylindrical sheet,said members being attached to said cylindrical sheet to provide asubstantially complete segmental cylindrical wall in reinforcement ofsaid cylindrical sheet.

1l. A device as specified in claim 10, in which the reinforcing membersare comprised each of a vertical section, forming part of the segmentalcylindrical Wall, a substantially horizontal arcuate section attachedalong the upper edge of said vertical section and providing an annularflange at the upper edge of the apron, which substantially completelycloses the upper part of 'the annular space contained between the wallsof the Vesseland the apron, to substantially reduce the losses ofvapoursby diffusion, and a second substantially horizontal arcuate sectionattached along the lower edge of said vertical section and directedtowards the center of the blanket.

12. A device as specified in claim 11 characterized in that thereinforcing members are stiifened into a corrugated shape and arereinforced with stiifening elements.

13. A device as specified in claim 1, in which the exible elasticplastic sheet material of both said blanket and said apron is asynthetic resin.

14. The combinationwith an oil storage tank comprising rigid side wallsand a fixed roof, a blanket comprising an upper thin sheet of exibleelastic plastic material and attached to the underside of said uppersheet, a plurality of small, contiguous, cellular float units,

said float units comprising an upper thin sheet of exible` elasticplastic material having attached thereto another thin Sheet of flexibleelastic plastic material in whicha plurality of contiguous hollow cellsare provided, these two iioat unit sheets being attached one to theother around said `cells to seal said cells individually and providesaid plurality of small contiguous cellular oat units, said -blanketbeing capable of floating on the free surface of the stored liquid withsaid first mentioned upper thin sheet at a small distance from and in aparallel direction with the free surface of the stored oilforprotecting'the same against evaporation, theblanket being vof a size andconfiguration to cover. substantially, but not entirely,

the entire lfree surface of the stored oil so as to leave only a narrowclearance between the periphery of the blanket and said side wallsproviding a narrow free annular liquid surface between said peripheryand said Walls whereby catching and wedging by unevennesses of saidwalls is substantially avoided when the blanket rises or falls with thelevel of the stored liquid; a flexible elastic apron for said blanketmade also of at least one thin sheet of flexible elastic plasticmaterial, which is of sufficient rigidity to 'be self-supporting, yetsufficiently flexible to be deformed if submitted to stresses resultingfrom being caught by unevennesses of said wall, and sufiiciently elasticto resume its original form when it has passed the unevennesses, saidapron extending upwardly from the blanket and fringing the blanketaround substantially its entire periphery in substantially wholly spacedrelation to said side walls for maintaining, above said narrow freeannular liquid surface and at said walls ya ring of hydrocarbon vapoursin equilibrium with the stored oil, the roof of said tank having atleast one vent affording open communication between the upper part ofthe tank and the atmosphere.

l5. An oil storage tank according to claim 14, in which said blanket andits apron consists essentially of unplasticized polyvinyl chloride inthe form of sheets or films of a thickness in the range of from a fewhundredths to a few tenths of a millimeter and in which said apron has aheight of at least 200 millimeters.

16. An oil storage tank in accordance with claim 15, in which said apronis of a structural rigidity sufficient to maintain the apron in an erectposition, and in which said blanket is of a construction to provideflotation cells at its underside in number and size sufiicient to floatsaid blanket on the oil at an elevation lower than ten millimeters.

17. A screen for protecting liquid contained in a storage vessel havingan upright wall and a fixed roof, comprising an upper thin sheet offlexible elastic plastic material and attached to the underside of saidupper sheet, a plurality of small, contiguous cellular float units, saidfloat units comprising an upper thin sheet of flexible elastic plasticmaterial having attached thereto a lower thin sheet of flexible elasticplastic material in which a plurality of contiguous hollow cells areprovided, these two float unit sheets being attached one tothe otheraround said cells to seal said cells individually and provide saidplurality of small, contiguous, cellular float units, said blanketybeing capa-ble of floating on the free surface of the stored liquid,and an apron of flexible elastic plastic material for said blanketdisposed in a generally upright position relative to said blanket andfringing said blanket around substantially its entire periphery, saidapron -being of a structural rigidity sufficient to maintain saidupright position when in use, yet sufficiently flexible to be deformedif submitted to stresses resulting from being caught by unevennesses ofsaid wall, and suflciently elastic to resume its original form when ithas passed the unevennesses, the material of both said apron and saidblanket being a synthetic resin, the different sheets of said materialbeing of a thickness in the range of from a few hundredths to a fewtenths of a millimeter, and said apron having a height of at least twohundred millimeters.

18. The combination with a storage vessel for volatile liquids, thevessel having an upright wall and a fixed roof, a blanket comprising athin sheet of flexible elastic plastic material and attached to one sideof said thin sheet, a plurality of small contiguous, cellular floatunits, said float units comprising a thin sheet of flexible elasticplastic material having attached thereto another thin sheet of flexibleelastic plastic material in which a plurality of contiguous hollow cellsare provided, these two float unit sheets being attached one to theother around said cells to seal said cells individually and provide saidplurality of small contiguous cellular float units, said blanket beingcapable of floating on the free surface of the stored liquid with theuppermost of said thin sheets at a small distance from and in a paralleldirection with the free surface of the stored liquid for protecting thesame against evaporation, the blanket being of a size and configurationto cover substantially, but not entirely, the entire free surface of thestored liquid so as to leave only a narrow clearance between theperiphery of the blanket and said wall providing a narrow free annularliquid surface between said periphery and said wall whereby catching andwedging by unevennesses of said wall is substantially avoided when theblanket rises or falls with the level of the stored liquid: and, anapron for said blanket, made also of at least one thin sheet of flexibleelastic plastic material, which is of sufl'icient rigidity to beself-supporting yet sufficiently flexible to be deformed if submitted tostresses resulting from being caught by unevennesses of said wall, andsufficiently elastic to resume its original form when it has passed theunevennesses, said apron extending upwardly from said blanket andfringing the blanket around substantially its entire periphery insubstantially Wholly spaced relation to said wall for maintaining, abovesaid narrow free annular liquid surface and at said wall, a ring ofvapours in equilibrium with the stored liquid, whereby the losses byevaporation on the free annular liquid surface are restricted in aconsiderable degree.

19. A screen for protecting liquid contained in a storage Vessel havingan upright wall and a fixed roof, comprising a thin sheet of flexibleelastic plastic material and attached to one side of said sheet, aplurality of small, contiguous cellular float units, said float unitscomprising a thin sheet of flexible elastic plastic material havingattached thereto a thin sheet of flexible elastic plastic material inwhich a plurality of contiguous hollow cells are provided, these twofloat unit sheets being attached one to the other around said cells toseal said cells individually and provide said plurality of small,contiguous, cellular float units, said blanket being capable of floatingon the free surface of the stored liquid, and an apron of flexibleelastic plastic material for said blanket disposed in a generallyupright position relative to said blanket and fringing said blanketaround substantially its entire periphery, said apron being of astructural rigidity sufficient to maintain said upright position when inuse, yet sufficiently flexible to be deformed if submitted to stressesresulting from being caught by unevennesses of said wall, andsufficiently elastic to resume its original form when it has passed theunevennesses, the material of both of said apron and said blanket beinga synthetic resin, the different sheets of said material being of athickness in the range of fro-rn a few hundredths to a few tenths of amillimeter, and said apron having a height of at least two hundredmillimeters.

References Cited in the file of this patent UNITED STATES PATENTS239,431 Birge Mar. 29, 1881 289,557 Reinecke Dec. 4, 1883 1,269,746Robeson .Tune 18, 1918 1,674,073 Stough June 19, 1928 2,014,264 PatrickSept. 10, 1935 2,023,308 Cantacuzene Dec. 3, 1935 2,281,748 Carney May5, 1942 2,330,366 Jayne Sept. 28, 1943 2,415,150 Stein Feb. 4, 19472,609,119 Shilstone Sept. 2, 1952 2,715,231 Marston Aug. 16, 19552,731,652 Bishop Ian. 24, 1956 FOREIGN PATENTS 832,842 France July 1l,1938

